[refactor/grpc] Add doc to mqtt adapter package
[refactor/grpc] Add doc to mqtt adapter package

TheThingsNetwork/ttn,LoRaWanSoFa/ttn,jvanmalder/ttn,mgranberry/ttn,mgranberry/ttn,jvanmalder/ttn,jvanmalder/ttn,TheThingsNetwork/ttn,mgranberry/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn